Global warming since 1995 'now significant'

Phil Jones's comments last year have become a touchstone for climate "sceptics"

Climate warming since 1995 is now statistically significant, according to Phil Jones, the UK scientist targeted in the "ClimateGate" affair.

Last year, he told BBC News that post-1995 warming was not significant - a statement still seen on blogs critical of the idea of man-made climate change.

But another year of data has pushed the trend past the threshold usually used to assess whether trends are "real".

Dr Jones says this shows the importance of using longer records for analysis.

It just shows the difficulty of achieving significance with a short time seriesPhil Jones

By widespread convention, scientists use a minimum threshold of 95% to assess whether a trend is likely to be down to an underlying cause, rather than emerging by chance.

If a trend meets the 95% threshold, it basically means that the odds of it being down to chance are less than one in 20.

Last year's analysis, which went to 2009, did not reach this threshold; but adding data for 2010 takes it over the line.

"The trend over the period 1995-2009 was significant at the 90% level, but wasn't significant at the standard 95% level that people use," Professor Jones told BBC News.

"Basically what's changed is one more year [of data]. That period 1995-2009 was just 15 years - and because of the uncertainty in estimating trends over short periods, an extra year has made that trend significant at the 95% level which is the traditional threshold that statisticians have used for many years.

"It just shows the difficulty of achieving significance with a short time series, and that's why longer series - 20 or 30 years - would be a much better way of estimating trends and getting significance on a consistent basis."

Professor Jones' previous comment, from a BBC interview in Febuary 2010, is routinely quoted - erroneously - as demonstration that the Earth's surface temperature is not rising.

Globally consistent

The dataset that Professor Jones helps to compile - HadCRUT3 - is a joint project between the Climatic Research Unit (CRU) at the University of East Anglia (UEA), where he is based, and the UK Met Office.

Phil Jones is back at the scientific helm of CRU, though relieved of administrative leadership

It is one of the main global temperature records used by bodies such as the Intergovernmental Panel on Climate Change (IPCC).

HadCRUT shows a warming 1995-2010 of 0.19C - consistent with the other major records, which all use slightly different ways of analysing the data in order to compensate for issues such as the dearth of measuring stations in polar regions.

Shortly before the UN climate summit in Copenhagen, Phil Jones found himself at the centre of the affair that came to be known as "ClimateGate", which saw the release of more than 1,000 emails taken from a CRU server.

Critics alleged the emails showed CRU scientists and others attempting to subvert the usual processes of science, and of manipulating data in order to paint an unfounded picture of globally rising temperatures.

Subsequent enquiries found the scientists and their institutions did fall short of best practice in areas such as routine use of professional statisticians and response to Freedom of Information requests, but found no case to answer on the charges of manipulation.

Since then, nothing has emerged through mainstream science to challenge the IPCC's basic picture of a world warming through greenhouse gas emissions.

And a new initiative to construct a global temperature record, based at Stanford University in California whose funders include "climate sceptical" organisations, has reached early conclusions that match established records closely.